A conventional chopping secondary mirror system is shown in, for example, the article "chopping secondary mirror for the ESO 3.6 m telescope" written by A. van Dijssenldonk and A.F.M. Moorwood in Proceedings of SPIE (The Society of Photo-Optical Instrumentation Engineering), SPIE Ostomeohanical Systems Engineering, Vol. 817, Pages 28-33 (1987).
In order to position the center of gravity of the chopping secondary mirror system, shown in this article, on the rotation axis of the secondary mirror, a balancing mass must be used exclusively for this purpose. This requires a space for the balancing mass, and when it is desired to use larger driving motors to increase power for driving the secondary mirror to chop, the balancing mass also becomes larger and complicated in shape, which disadvantageously causes the overall size of the secondary mirror system to increase.
Another problem is that the secondary mirror could be deformed due to heat dissipated by a coil arrangement and also due to the weight of the chopping secondary mirror section, which in turn causes reduction of precision of astronomical observation.
This conventional system has still another problem. The conventional system employs flexural pivots in a reaction force compensator which receives reaction force generated when the secondary mirror is oscillated by the driving motors. Springs which form the flexural pivots are very expensive, and, furthermore, they are too flexible to damp vibrations caused by reaction force which is caused by the driving motor action.
An object of the present invention is to provide a chopping secondary mirror system which can be formed small in size and which is free of the above mentioned disadvantages.
Another object of the present invention is to provide a chopping secondary mirror system which has minimum deformation of the secondary mirror caused by, for example, the weight of the chopping secondary mirror section and heat generated by constituent components.
Still another object of the present invention is to provide a chopping secondary mirror system in which vibrations caused by reaction force given by driving motors are efficiently reduced by means of a reaction force compensator, and which can still be manufactured at low cost.